The present invention relates to an actuator system having an output shaft which is held in a first position by a spring and upon command an electric motor drives the shaft to a second position against the force of the spring and holds the shaft in the second position until a further command allows the spring to return the shaft to the first position.
The present invention overcomes these problems by noting that the current in the motor increases sharply when the shaft reaches the stalled position. Thus, detecting the sharp rise in current becomes a simple way to detect the stalled condition, and does not involve waiting for a time period as great or greater than the normal period for one complete revolution to determine the condition, as may be required if only a rotation sensor is relied on. The problem of possible gear damage is overcome in the present invention, with the use of a current limiting circuit to prevent the current in the motor from increasing beyond a predetermined limit chosen so that the gear train will not be damaged. This satisfactorily protects the gear train in a high temperature embodiment of the present invention, which uses the Hall effect devices to sense rotation of the output. However, in a low or normal temperature embodiment of the present invention, the Hall effect devices are not needed but, in order to detect a stalled condition, with a current limiter in operation, the voltage to the motor is rapidly stopped and started so that the current repeatedly and rapidly oscillates just below the current limiting value. A counter is employed to count these oscillations and a predetermined number of such oscillations becomes an indication that the motor is stalled.